Catalytic electrodes having a valve metal base and a catalytic coating comprising at least one platinum-group metal oxide are described for example in the following patents:
U.K. Pat. No. 1,147,442 PA1 U.K. Pat. No. 1,195,871 PA1 U.K. Pat. No. 1,231,280 PA1 U.K. Pat. No. 1,235,570 PA1 U.S. Pat. No. 3,687,724 PA1 (i) A semi-conducting, insoluble, stable polymer matrix is formed directly in situ on the substrate surface, by controlled application of a predetermined polymer containing liquid composition, followed by controlled heat treatment. PA1 (ii) The catalyst simultaneously formed in situ is uniformly distributed throughout the semi-conducting polymer matrix so as to provide a consolidated coating of uniform composition. PA1 (iii) This uniform distribution thus allows the catalyst to be used as effectively as possible, i.e. a minimum amount of platinum-group metal catalyst needs to be incorporated in the coating, only in order to provide adequate catalytic properties. PA1 (iv) On the other hand, the semi-conducting polymer matrix itself provides adequate current conduction and uniform current distribution throughout the coating, thereby allowing it to support high current densities. PA1 (v) The semi-conducting insoluble polymer matrix is moreover relatively stable and resistant to both physical and electrochemical attack, and thus may serve as a semi-conducting protective binder for the catalyst, while at the same time protecting the underlying substrate and promoting adherence of the coating to the substrate. PA1 (vi) The above advantages may more particularly provide corrosion resistant dimensionally stable electrodes of the invention with stable electrochemical performance and a long useful life under severe operating conditions.
Dimensionally stable anodes of the type described in U.K. Pat. 1,195,871 are currently used for the electrolytic production of chlorine and provide a high anode life with reduced energy consumption.
However, when such anodes are used in metal electrowinning processes where oxygen is produced at the anode, they tend to undergo passivation and thus have a relatively short useful life. Passivation seems to be caused in this case by diffusion of oxygen through the coating resulting in build-up of an insulating oxide layer on the valve metal substrate and/or dissolution of ruthenium in the surrounding acid medium. In U.S. Pat. Re. 29,419, a coating is described which is especially intended as an anode for electrowinning. Finely divided ruthenium dioxide used as an electrocatalyst is applied together with an organic polymer to a substrate, followed by curing to produce a coating comprising the electrocatalyst dispersed in an inert polymer matrix. The organic polymer is intended in this case to serve as a binder, to provide mechanical support for the electrocatalyst to provide adhesion to and protection of the underlying substrate. According to the application intended, the amount of electrocatalyst used per part by weight of the polymer may generally vary within the range from 6:1 to 1:1, in order to provide the desired properties of such a coating. The conductive properties of such a composite coating will thus depend essentially on the amount of electrocatalyst dispersed in the inert polymer matrix, as well as its size and distribution throughout the matrix. The electrocatalyst must moreover be prepared in the form of extremely fine particles of less than 0.1 micron size, and then applied together with the organic polymer as uniformly as possible in order to be able to ensure the desired properties of the coating.
The state of the art relating to composite electrodes comprising conductive fillers dispersed in a polymer matrix may be further illustrated by U.S. Pat. Nos. 3,629,007, 3,751,301 and 4,118,294. The German patent application Offenlegungsschrift 2035918 further relates to an electrode for electrolytic purposes, which comprises a metallic support coated with an organic semi-conductor consisting of a polymeric metal phthalocyanine. U.S. Pat. Nos. 3,792,732, 3,881,957 and 4,090,979 moreover relate to catalytic electrode materials comprising semi-conducting polymers formed on a refractory oxide and impregnated with metal catalysts.
The production of satisfactory catalytic coatings for dimensionally stable electrodes presents in fact many complex problems due to the severe technical and economic requirements of industrial electrolytic processes. These problems relate on one hand to the choice of suitable materials to provide adequate catalytic activity and conductivity, as well as being physically and electrochemically stable under the severe operating conditions of industrial electrolytic processes, such as high current density, corrosive media and products. Good electrocatalysts such as the platinum-group metal oxides are moreover very expensive so that they must generally be used in minimum amounts and combined with cheaper inert coating materials which, however, are generally catalytically inactive and electrically non-conductive (e.g. TiO.sub.2 and most organic polymers). On the other hand, many potentially interesting electrode materials may be unsuitable for the manufacture of satisfactory coatings which meet all requirements of industrial electrolytic processes. Thus, for example, various organic semi-conductors of potential interest as electrode materials, are relatively difficult to manufacture and moreover are difficult to process into satisfactory coatings.
It is an object of the present invention to provide an electrocatalytic coating comprising at least one platinum-group metal catalyst forming a uniform solid mixture with an insoluble semi-conducting polymer of an electroconductive substrate. Another object of the invention is to provide a simple process for the industrial manufacture of electrocatalytic coatings comprising at least one platinum-group metal catalyst and an insoluble semi-conducting polymer forming a uniform mixture on an electroconductive substrate.